This invention relates to methods and apparatuses for the analysis of patient""s coughs. More specifically, this invention relates to methods and apparatuses for the analysis of patient""s coughs to aid in diagnosing pulmonary disorders and diseases. This method uses signal analysis techniques to extract quantitative information from recorded cough sound pressure waves. Moreover, the method allows the recordation of cough sound waves while avoiding distortions caused by reflections. The generated data can be used to diagnose pulmonary disorders and diseases as well as track the effectiveness of treatment regimes over time. The method can also be used for screening the general population, or populations at higher risk, so that such pulmonary disorders and diseases can be detected as early as possible so that appropriate treatment can be started as soon as possible.
Cough is associated with well over 100 different pulmonary diseases and is one of the most common signs or symptoms of respiratory disease. Even though cough may be an unwanted complication of a pulmonary disease, it has often been used by physicians as an effective diagnostic tool. Since cough sounds are composed of acoustic information which can be altered by lung disease and since cough has essentially the same acoustical characteristics whether performed voluntarily or involuntarily, analysis of voluntary cough sounds has the potential to become a useful noninvasive tool for screening large populations of workers to evaluate their pulmonary function. The use of cough sound analysis to aid in the identification of lung disease has several distinct advantages since testing can be quickly and easily administered while requiring only a minimum amount of technician or patient training.
In order to describe the events that occur during a cough, physiologists have subdivided a cough into 4 different phases (Leith et al., Cough, In: The Handbook of Physiology, The Respiratory System edited by A. Fishman, P. T. Macklem and J. Mead, Bethesda, Md., Am. Physiological Society, Sec(3) Chapter 20, 315-336 (1987)). During the initial phase, called the inspiration phase, a variable volume of air is inhaled into the lungs. The second phase, referred to as the compression phase, begins as the glottis closes and the muscles of expiration begin to contract increasing thoracic pressure. The third phase is called the expulsion phase. At the start of the third phase, the glottis opens and gas flows rapidly from the lung. During the fourth and final phase, called the cessation phase, muscle activity is reduced and airflow is diminished.
The physical characteristics of a cough are illustrated in FIG. 1. Flow from the mouth during a cough is shown in FIG. 1A. Positive values represent flow from the lungs while negative flow values indicate air flow into the lungs. During the initial phase of a cough (phase I) air flow is negative as air enters the lungs. The volume of air inspired is variable and is said to be a function of the anticipated forcefulness of the cough (Yanagihara et al., xe2x80x9cThe Physical Parameters of Cough: the Larynx in a Normal Single Cough,xe2x80x9d Acta. Oto-laryngol. 61: 495-510 (1966)). As compression of air occurs during phase II of the cough, the glottis closes and airflow ceases. When the glottis reopens, in approximately 200 ms, flow initially increases and then decreases rapidly creating a flow transient. This initial rapid change in flow during phase III is referred to as supramaximal flow and is thought to result from the air rapidly leaving the flexible airway system as the airways compress during the initial part of the expulsion phase of a cough. At the same time that air is leaving the airways during the initial portion of phase III, expiratory flow from the lung periphery rises sharply to maximal flow which is limited by the maximum expiratory flow-volume relationship that is unique for each lung. Air flow leaving the lungs during a cough, therefore, is a summation of the transient air leaving the airways at a supramaximal flow rate and the air leaving the periphery of the lungs at maximal flow. During the cessation phase IV of a cough, airflow from the lungs diminishes and then approaches zero as muscle activity decreases.
FIG. 1B illustrates a typical sound pressure wave generated by a cough. It has been suggested that the cough sounds are generated during phase III and sometimes during phase IV of a cough. The cough sound, itself, can be subdivided into two and sometimes three parts (Thorpe et al., xe2x80x9cTowards a Quantitative Description of Asthmatic Cough Sounds,xe2x80x9d Eur. Respir. J. 5: 685-692 (1992)). The first part of a cough sound is referred to as the initial burst and represents the sound transient that is associated with the glottis opening. The second or middle part corresponds to the interval of near steady, maximal flow coming from the periphery of the lung which occurs with the glottis maximally open. The third part of a cough, called the final burst, is not always present, but is believed to occur in some subjects who close their glottis during the cessation phase of a cough.
Airflow from the lung during a cough and the maximum expiratory flow volume (MEFV) relationship of a lung have much in common. During a forced expiration the airways, which are very flexible cylindrical structures, undergo compression and decrease in cross-sectional area as air rapidly passes through them. As a result, one or more choke points is created in the airway system during maximal gas flow. After a choke point has formed, flow from the lungs becomes independent of the driving pressure. This is important because it implies that airflow through the airway system should become effort independent during the performance of a MEFV maneuver. Once effort independence is reached, the MEFV relationship becomes repeatable. Flow-volume curves recorded during a MEFV maneuver define the limits of flow and volume that can be achieved during most expiratory maneuvers in a given individual. Leith et al. (1987) have stated that a surprisingly modest expiratory effort is required to reach the outer limits of the flow-volume domain for a given individual, making forced expiration a reliable pulmonary function test. FIG. 2 shows an example of an MEFV curve while expiring with a maximum effort into a spirometer. An example of the flow volume relationship of a lung during a cough is superimposed on the MEFV curve in FIG. 3. During the initial phase of a cough, air is inspired into the lungs. This is indicated by the increase in lung volume as the operating point on the flow-volume curve moves to the left throughout phase I. During the compression phase, there is no gas flow so phase II is represented by a single point on the diagram. During the initial part of phase III, a supramaximal flow transient is observed as the volume of air in the flexible airways decreases quickly as the airways begin to collapse. Following the very brief flow transient, maximal flow is achieved which approaches the maximal flow reached during the performance of a MEFV maneuver. The events that occur during this portion of the cough are very similar to those that occur during a MEFV maneuver; therefore, it can be assumed that airflow leaving the lung during a cough reaches flow limitation and is reasonably reproducible when successive coughs are performed beginning at the same lung volume. Since the mechanisms producing cough sounds are dependent upon air flow, it seems likely that cough sounds are also reproducible if similar lung histories are followed prior to each cough.
A block diagram of a simple model illustrating how cough sounds are produced is shown in FIG. 4. It is thought that the acceleration and turbulence of air within the airways caused by the rapid expulsion of air from the lungs generates band limited noise which is then modified by the resonances of the lungs"" upper airways and possibly the oral and nasal cavities as air travels toward the mouth. Peaks that occur in the spectra of cough sounds result from resonances along the cough sound pathway, and they are similar to the formants observed in speech analysis. A second source of sound is referred to as a wheeze and is thought to result from the fluttering of airway walls as gas moves rapidly through the airways. A similar model has previously been proposed for the study of unvoiced speech (Oppenheim et al., xe2x80x9cThe Speech Modelxe2x80x9d in Discrete-time Signal Processing, New Jersey, Prentice Hall, Chapter 12, 816-825 (1989)).
In the past, several groups of investigators have recorded cough sounds in a variety of ways and have attempted to develop a technique which could be used to show differences in cough sounds between healthy subjects and those with respiratory diseases. It was thought that any substantial differences between coughs could eventually become useful in identifying persons with respiratory diseases. These initial studies examined the coughs of subjects with a variety of obstructive lung diseases, but the most often studied population was those having asthma.
Debreczeni et al. (xe2x80x9cSpectral Analysis of Cough Sounds Recorded with and Without a Nose Clip,xe2x80x9d Bull. Eur. Physiopathol. Respir. Suppl. No. 10, 57s-61s (1987); xe2x80x9cSpectra of the Voluntary First Cough Sound,xe2x80x9d Acta Physiol. Hung. 75(2): 117-131 (1990)) recorded sound pressure waves and computed the average spectra of cough sounds from patients with several types of obstructive lung disease. The spectra were used to determine how cough sound energy was distributed within several frequency bands and to distinguish between coughs from healthy subjects and those subjects with lung disease.
Piirila et al. (xe2x80x9cDifferences in Acoustic and Dynamic Characteristics of Spontaneous Cough in Pulmonary Diseases,xe2x80x9d Chest 96: 46-53 (1989)) recorded cough sound pressure waves and airflow for a sequence of coughs and then computed average cough spectra. The peak values representative of the dominant frequency components within the cough were extracted for comparison. Spectrograms were also computed and the length of the cough maneuver was measured by determining the time that sound energy had a frequency component present at 500 Hz. They reported that cough sound duration was longer for asthmatic coughs than for coughs from control subjects. One problem with interpreting these studies, however, is that sequences of coughs were studied instead of single coughs. As a result, it has been difficult to interpret the measurements and conclusions of this study concerning the duration of a cough.
Researchers have studied several aspects of recording and interpreting cough sounds. Initially, waterfall plots of cough spectrograms were examined to determine if there were differences in cough sounds of children with asthma before and after exercise (Toop et Al., xe2x80x9cCough Sound Analysis: A New Tool For the Diagnosis of Asthma?xe2x80x9d, Family Pract., 6(2): 83-85 (1989)). More recently, attempts have been made to evaluate cough sounds using more quantitative methods so that meaningful comparisons could be made between cough sounds of selected individuals (Thorpe et al., xe2x80x9cTowards a Quantitative Description of Asthmatic Cough Sounds,xe2x80x9d Eur. Respir. J. 5: 685-692 (1992)). The cough sound was divided into two or three parts and the characteristics of each part was studied independently. Power spectra were computed, normalized, and treated as histograms. The mean frequency, standard deviation, skewness, and kurtosis were calculated. In addition, the energy within selected frequency bands was examined and differences in coughs from persons with several types of obstructive lung disease were noted. This study also examined features of the sound pressure wave with respect to time including the duration, root mean square (RMS) value and zero crossing rate. Interestingly, no significant difference in the total duration of a cough between healthy subjects and those with obstructive lung diseases was reported. They did note, however, that both the duration of the initial burst and zero crossing rates of the cough waveform during each of the first two phases were smaller for asthmatic than for non-asthmatic coughs.
A variety of methods have been used to record and digitize cough sounds for analysis. The methods that have been used, however, have a large influence on the quality of the signal used for the acoustical analysis. Debreczeii, et al. (1987, 1990) recorded cough sounds of a seated subject in a quiet room with a microphone directed towards the subject""s mouth from a distance of 50 cm. Sounds were recorded on an analog tape recorder and then digitized at a rate of 5 kHz and 20 kHz. Piirila et al. (1989) recorded cough sounds from subjects in a sitting position with a microphone attached to the skin of their chest wall located over the sternal manubrium. The sounds were recorded with a tape recorder; the bandwidth of their spectral analysis was 9 kHz. Toop et al. (xe2x80x9cA portable system for the spectral analysis of cough sounds in asthma,xe2x80x9d J. of Asthma 27(6) 393-397 (1990)) described the design of a system used to record cough sounds. A patient coughed into a tube with a pneumotach and microphone attached. The cough signal was digitized at 5 kHz for analysis with a personal computer which limited the bandwidth of their analysis to frequencies below 2.5 kHz. Since the pneumotach modified the characteristics of the sound pressure wave reaching the microphone, these investigators estimated the correct acoustical response by deconvolving their average spectral measurements by Weiner filtering.
In spite of past efforts, it would be desirable to provide a simple, reliable, and fast diagnostic method to analyze coughs in order to assist physicians in diagnosing lung disorders or diseases. It is also desirable to provide a simple, reliable, and fast diagnostic method to analyze coughs which will allow physicians to monitor the effectiveness of treatments prescribed for lung disorders or diseases. It is also desirable to provide a simple, reliable, and fast diagnostic method to analyze coughs which can be used to screen populations for lung disorders or diseases, especially in cases where such lung disorders or diseases can be detected in an early stage where treatments can be more effectively administered and damage to lung function can be avoided or minimized. The methods and apparatuses of this invention provide such diagnostic methods.
This invention provides a fast, simple, reliable method and apparatus for recording cough sounds for diagnostic and other medical purposes. More specifically, this invention relates to methods and apparatuses for the analysis of patient coughs to aid in diagnosing pulmonary disorders and diseases. This method uses signal analysis techniques to extract quantitative information from recorded cough sound pressure waves. The generated data can be used to diagnose pulmonary disorders and diseases as well as track the effectiveness of treatment regimes over time. The method can also be used to quickly and reliably screen individuals at risk of pulmonary disorders and diseases. The discovery of early stages of pulmonary disorders or diseases may allow earlier treatment and/or environmental modification to reduce the risk of irreversible injury to pulmonary function.
The present invention provides a method for analyzing coughs for diagnostic purposes. This invention also provides a system for recording high fidelity cough sound measurements. Moreover, this invention provides a simple, non-invasive system that can quickly and easily be administered with minimum technician and patient training. The system comprises a mouthpiece, a tube having a distal end and a proximal end, a flexible tubing having a distal end and a proximal end, and a microphone; wherein the mouthpiece is attached to the proximal end of the tube, wherein the distal end of the tube is attached to the proximal end of the flexible tube, wherein the microphone is attached to the tube between its distal and proximal ends such that the microphone can record sound pressure waves within the system without distorting the pressure waves, and wherein the flexible tubing is sufficiently long so there are essentially no reflected sound pressure waves which interfere with the recording of the sound pressure waves at the microphone. Preferably, the system also includes a computer system to assist in recording and analyzing the sound pressure waves. Preferably, the distal end of the flexible tubing has an anechoic termination to further reduced or attenuate reflected sound waves.
This invention also provides a method for analyzing a patient""s cough for diagnostic purposes, said method comprises (1) providing a system for analyzing coughs wherein the system comprises a mouthpiece, a tube having a distal end and a proximal end, a flexible tubing having a distal end and a proximal end, and a microphone; wherein the mouthpiece is attached to the proximal end of the tube, wherein the distal end of the tube is attached to the proximal end of the flexible tube, wherein the microphone is attached to the tube between its distal and proximal ends such that the microphone can record sound pressure waves within the system without distorting the pressure waves, and wherein the flexible tubing is sufficiently long so there are essentially no reflected sound pressure waves which interfere with the recording of the sound pressure waves at the microphone; (2) allowing the patient to cough into the mouthpiece; (3) recording the sound pressure waves generated by the patient""s cough with the microphone; and (4) analyzing the recorded sound pressure waves. Preferably, the recorded sound pressure waves are digitized and then analyzed. Preferably, the recorded sound pressure waves are analyzed using spectrograms from which contour plots can be generated.
These and other objectives and advantages of the present invention will be apparent to those of ordinary skill in the art upon consideration of the present specification.